How to Calculate U-Value: Step-by-Step Guide

Updated 11 November 2026 · Standard formula, shown below

This guide explains how to calculate the U-value (thermal transmittance) of a building element like a wall, roof, or floor. You’ll learn the formula, what data you need, and how to apply it correctly, with a worked example using standard material values.

What You’ll Need

Thermal conductivity (k-value) for each material layer, in W/(m·K). These are found in material datasheets or standard tables.
Thickness of each layer, in meters (convert from mm by dividing by 1000).
Surface resistance values (internal and external) – standard values are often used, e.g., Rsi = 0.13 m²K/W and Rse = 0.04 m²K/W for vertical surfaces.
A calculator or spreadsheet.

Step-by-Step

List all layers – Write down every layer from inside to outside. For example: plaster, brick, insulation, brick.
Get the k-value for each layer – Find the thermal conductivity (W/(m·K)) from a reliable source. Common values: brick ~0.6–0.8, insulation (mineral wool) ~0.035–0.045, plaster ~0.4–0.6.
Convert thickness to meters – If a layer is 100 mm, write it as 0.1 m. This is a common unit error.
Calculate each layer’s R-value – For each layer, divide thickness by k-value: R_layer = thickness / k. The unit is m²K/W.
Add surface resistances – Add the internal (Rsi) and external (Rse) surface resistances. For walls, use Rsi = 0.13 and Rse = 0.04. For roofs, values differ (e.g., Rsi = 0.10, Rse = 0.04).
Sum all R-values – Add all layer R-values plus the two surface resistances to get the total R_total.
Take the reciprocal – U-value = 1 / R_total. The result is in W/(m²K).
Check if your U-value meets a target – Compare with building regulations or design goals. Lower U-value means better insulation.

Formula

U = 1 / (Rsi + Σ(R_layer) + Rse)

Where:

U = thermal transmittance (W/(m²K))
Rsi = internal surface resistance (m²K/W) – typically 0.13 for walls
Rse = external surface resistance (m²K/W) – typically 0.04 for walls
R_layer = thickness / k for each layer (m²K/W)
Σ means sum of all layers

Worked Example

Calculate the U-value of a wall with these layers (inside to outside):

Plaster: 13 mm (0.013 m), k = 0.5 W/(m·K)
Brick: 100 mm (0.1 m), k = 0.6 W/(m·K)
Insulation: 50 mm (0.05 m), k = 0.035 W/(m·K)
Brick: 100 mm (0.1 m), k = 0.6 W/(m·K)

Step 1: Calculate each R:

Plaster: 0.013 / 0.5 = 0.026
Brick: 0.1 / 0.6 = 0.1667
Insulation: 0.05 / 0.035 = 1.4286
Brick: 0.1 / 0.6 = 0.1667

Step 2: Add surface resistances: Rsi = 0.13, Rse = 0.04

Step 3: Total R = 0.13 + 0.026 + 0.1667 + 1.4286 + 0.1667 + 0.04 = 1.958

Step 4: U = 1 / 1.958 = 0.511 W/(m²K)

So the U-value is approximately 0.51 W/(m²K).

Common Mistakes

Unit errors: Forgetting to convert mm to meters. 100 mm is 0.1 m, not 100.
Using k-value instead of R-value: You must divide thickness by k, not multiply.
Omitting surface resistances: These are always present and affect the result, especially for well-insulated assemblies.
Mixing up Rsi and Rse: Internal and external values are not interchangeable; use the correct ones for your orientation.
Averaging k-values incorrectly: You cannot simply average k-values of layers; you must sum R-values.

Tips

Always use consistent units – work in meters and W/(m·K).
For existing buildings, measure actual thicknesses, not just design drawings.
Use standard surface resistances from your local building code for consistency.
Round only at the final step to avoid rounding errors.
For multi-layer walls with air gaps, include the R-value of the air gap (typically 0.18 for a ventilated gap).

Troubleshooting

Your U-value seems too high or too low. Check the k-values – they vary significantly by material density and moisture. Also verify thickness conversions.

Result is negative? You likely subtracted instead of dividing. R = thickness / k, not k / thickness.

You get a very large number? Check if you used mm instead of meters in the thickness. For example, 100 mm divided by 0.04 gives 2500, which is wrong.

Missing a layer? Make sure you include every layer, including air gaps and membranes. Forgetting a layer can change the U-value by 10% or more.

Not sure about k-values? Use conservative estimates from standard tables, or consult the material manufacturer’s data.

Frequently Asked Questions

What is a good U-value? A good U-value depends on your climate and building code. In cold climates, walls often target 0.3 W/(m²K) or lower; roofs lower. Lower is better for insulation.

What is the difference between U-value and R-value? R-value measures resistance to heat flow (higher is better), while U-value measures heat transmittance (lower is better). U = 1/R for a single layer, but for assemblies, U = 1/total R.

Do I need to include surface resistances? Yes, for accurate results. They account for heat transfer at the surfaces and are part of the standard calculation.

Can I calculate U-value for windows? Yes, but windows are more complex due to frames, glazing, and air gaps. This method works for simple opaque assemblies; for windows, use manufacturer data or specialised software.

How does insulation thickness affect U-value? Increasing insulation thickness increases total R, thus decreasing U-value. The effect is linear in terms of R, but the U-value change is not linear – it diminishes as insulation gets thicker.

What if I have a layer with unknown k-value? Use a typical value from tables for that material type, or measure the R-value directly with a heat flow meter. Avoid guessing wildly.

Method: the standard formula for this calculation, rounded as each result states. See how the calculators work or report an error.